Mugen MbX5 spur.

[MetalMan]

There is no one-piece spur gear solution, but there is another way. Mugen makes an adapter plate for their diffs that allow an Ofna spur gear like this one to mount onto the diff:
http://www3.towerhobbies.com/cgi-bin...?&I=LXBM16&P=7
I think someone mentioned that the gear will need some modifications to fit. Unfortunately I cannot find the Mugen spur gear adapter part number, but it's out there.

The normal solution is a Kyosho plastic spur gear. That doesn't work because normal 1/8 diffs have a 6mm output shaft, but Mugen diffs use a 5mm output.

As long as you use a hardened pinion, there really isn't any problem running a steel spur with a steel pinion. Many of us have had great luck doing so.